A data center may be defined as a location, for instance, a room that houses computer systems arranged in a number of racks. A standard rack, for example, an electronics cabinet, is defined as an Electronics Industry Association (EIA) enclosure, 78 in. (2 meters) high, 24 in. (0.61 meter) wide and 30 in. (0.76 meter) deep. These racks are configured to house a number of computer systems, about forty (40) systems, with future configurations of racks being designed to accommodate 200 or more systems. The computer systems typically include a number of printed circuit boards (PCBs), mass storage devices, power supplies, processors, micro-controllers, and semi-conductor devices, that dissipate relatively significant amounts of heat during their operation. For example, a typical computer system comprising multiple microprocessors dissipates approximately 250 W of power. Thus, a rack containing forty (40) computer systems of this type dissipates approximately 10 KW of power.
The power required to transfer the heat dissipated by the components in the racks to the cool air contained in the data center is generally equal to about 10 percent of the power needed to operate the components. However, the power required to remove the heat dissipated by a plurality of racks in a data center is generally equal to about 50 percent of the power needed to operate the components in the racks. The disparity in the amount of power required to dissipate the various heat loads between racks and data centers stems from, for example, the additional thermodynamic work needed in the data center to cool the air. In one respect, racks are typically cooled with fans that operate to move cooling air across the heat dissipating components; whereas, data centers often implement reverse power cycles to cool heated return air. The additional work required to achieve the temperature reduction, in addition to the work associated with moving the cooling fluid in the data center and the condenser, often add up to the 50 percent power requirement. As such, the cooling of data centers presents problems in addition to those faced with the cooling of the racks.
Conventional data centers are typically cooled by operation of one or more air conditioning units. For example, compressors of air conditioning units typically consume a minimum of about thirty (30) percent of the required operating energy to sufficiently cool the data centers. The other components, for example, condensers and air movers (fans), typically consume an additional twenty (20) percent of the required total operating energy. As an example, a high density data center with 100 racks, each rack having a maximum power dissipation of 10 KW, generally requires 1 MW of cooling capacity. Air conditioning units with a capacity of 1 MW of heat removal generally requires a minimum of 300 KW input compressor power in addition to the power needed to drive the air moving devices, for instance, fans and blowers. Conventional data center air conditioning units do not vary their cooling fluid output based on the distributed needs of the data center. Instead, these air conditioning units generally operate at or near a maximum compressor power even when the heat load is reduced inside the data center.
The efficiencies at which the air conditioning units are able to cool the data centers are functions of the temperature of heat addition and the temperature of heat rejection (Carnot power cycle). The efficiency (η) of a classic Carnot power cycle is derived from the following equation:
      Equation    ⁢                  ⁢    1    ⁢          :        ⁢                  ⁢    η    =      1    -                  T        heatrejection                    T        heataddition            
As seen in the equation above, as the temperature of heat addition rises, the efficiency increases. The efficiency also increases as the temperature of heat rejection to the environment decreases.
A common type of heat extraction system employed in data centers includes reverse power cycle systems, which are also known as vapor-compression cycles. In reverse power cycle systems, heat addition occurs in the evaporator and heat rejection occurs in the condenser. A pressure (P)—enthalpy (h) diagram 600 depicting a typical vapor-compression cycle for heat rejection from data centers using R134a refrigerant is illustrated in FIG. 6A. In the diagram 600, heat addition (Qevap) occurs in the evaporator (C-D), work input (Wc) occurs at the compressor (D-A), and heat rejection (Qcond) occurs at the condensor (A–B). The processes C–D and A–B occur at constant temperatures and are referred as evaporator temperature (Tevap) and condenser temperature (Tcond), respectively.
Heat extraction from data centers occurs at the evaporators (Qevap) of air conditioning units. Heat rejection occurs at the condensers (Qcond) of the air conditioning units and is the sum of the compressor work (Wc) and the evaporator heat addition (Qevap). The coefficient of performance (COP) of air conditioning units is the ratio of desired output (Qevap) over the work input (Wc), that is:
      Equation    ⁢                  ⁢    2    ⁢          :        ⁢                  ⁢    COP    =                    Q        evap                    W        c              .  
The COP of air conditioning units is improved by reducing the required compressor work (Wc) to provide the same amount of cooling (i.e., Qevap). This is graphically illustrated in the COP versus condenser temperature (Tcond) plot 602 depicted in FIG. 6B. The COP results depicted in FIG. 6B are based on an evaporator temperature of 10° C. and a compressor isentropic efficiency of 60%. Because heat can only be rejected to the ambient surroundings over a negative temperature gradient, the ambient temperature gates the temperature of heat rejection to the external environment (i.e., condenser temperature). Accordingly, ambient temperatures place a theoretical limit on the maximum efficiency of data center air conditioning systems.